I live in a rental home that has a Lennox g-15 furnace and an outside air unit. I previously had an old mercury thermostat that went bad last summer. I went to Home Depot and purchased a new one and thought I would replace it myself. Well, I didn't color code my wires and ended up not getting it to work right. I called a professional hvac person to fix it and he got the a/c running fine. Since it was summer, I didn't think to check if the furnace was coming on. Well, just my luck, it does not work. The old home has baseboard elec. Heaters, but on windy days they just don't keep it warm enough. When I turn the thermostat to heat, after about 10 seconds the fan starts to blow, but the gas never kicks in. The furnace was working fine prior to the thermostat problem. The gas is on, the pilot is lit, but no heat. On the thermostat itself, there are five wires coming out of the wall, but only four are connected. It appears to be either blue or green. I sincerely appreciate any help you can offer, because I am disabled, and simply can't afford to call a service man. Thank you so much for your help... MAC

What model tstat do you have? Please provide the wire color and terminal label they are attached to? We love pics.

If the HVAC guy didn't test your furnace then that's inexcuseable in my book! If the HVAC guy did test it and it worked just fine, you could be looking at something that has busted in the meantime.